Near-field observation of a surface-plasmon wave interacting with a three-dimensional wedge-shaped structure

We experimentally investigate the near field of a surface-plasmon wave that is interacting with a three-dimensional wedge-shaped resist structure using phase resolved near-field scanning optical microscopy. Upon entering the resist structure the surface-plasmon wave is exposed to a continuously increasing effective refractive index. As a consequence the dispersion relation of the surface plasmon varies accordingly. Similar to the shoaling of tsunami waves that are entering shallow water, the wavelength and the group velocity of the surface plasmon are reduced while the in-plane electric field component simultaneously increases. We provide measurements, simulations, and a theoretical analysis of the described behavior.